I've just watched this footage of SLS's Mobile Launcher Platform being rolled out to LC-39B for fitting tests.
On the front of the drive trucks are two rotating discs with red markings which are clearly visible around 00:06 in the video that I haven't noticed before. I presume they are some component of the drive mechanism, but I can't work out what.
What are they and what is their function?
They are very reminiscent of disc brakes; the blue objects look a lot like brake calipers. However, if this is the case, I'm unsure why they're totally exposed rather than within the structure. For cooling?